The present invention relates to a method for controlling a matrix converter, in particular a matrix converter with nine bidirectional power switches arranged in a 3xc3x973 switch matrix.
A matrix converter is a self-commutated direct converter. It enables the conversion of a constant three-phase system into a system with variable voltage and frequency. Through the arrangement of the bidirectional power switches in a 3xc3x973 switch matrix, each of the three output phases of the matrix converter can be electrically connected to any one input phase. One phase of the matrix converter includes an arrangement of three bidirectional power switches wherein each switch is connected, on the one hand, to an input phase and, on the other hand, to an output phase. An arrangement of this type is also referred to as a 3xc3x971 switch matrix. The matrix converter does not require an intermediate circuit. Due to its topology, the self-commutated direct converter advantageously has a recovery capability and achieves sinusoidal mains currents through a suitably designed control.
Each of the bidirectional power switches of the matrix converter has two anti-serially connected semiconductor switches. Insulated Gate Bipolar Transistors (IGBT) are preferably used as semiconductor switches, which each include an antiparallel diode. Bidirectional power switches designed in this way are preferably used in converters for small and medium power. Through the control of these semiconductor switches of the bidirectional power switches, a continuous current path is established in a direction determined by the arrangement of the semi-conductor switches. If both semiconductor switches of a bidirectional power switch are controlled, the latter is bidirectionally activated and a current can flow in both directions. This creates a safe electrical connection between an input phase and an output phase of the matrix converter. If only one semiconductor switch of a bidirectional power switch is controlled, the latter is unidirectionally activated, creating an electrical connection between an input phase and an output phase of the matrix converter only for a preferred current direction.
Any desired time-averaged output voltage can be obtainedxe2x80x94within certain limitsxe2x80x94by a controlled temporal sequence of combinations of switch positions within a modulation period. A matrix converter includes a controller capable of computing a suitable switch combination based on information about the input voltage space vector and a desired value for the output voltage space vector.
Conventional control methods operate either according to a phase-oriented or a vector-oriented method.
The phase-oriented control method is described in the publication xe2x80x9cAnalysis and Design of Optimum-Amplitude Nine-Switch Direct AC-AC Convertersxe2x80x9d, by Alberto Alesina and Marco G. B. Venturini, IEEE Transactions on Power Electronics, Vol. 4, No. 1, January 1989, pp. 101-112. The space vector control method is described in xe2x80x9cSpace Vector Modulated Three-Phase to Three-Phase Matrix Converter with Input Power Factor Correctionxe2x80x9d, by Lxc3xa1szxc3x3 Huber and Du{haeck over (s)}an Borejevixc4x87, IEEE Transactions on Industrial Applications, Vol. 31, No. 6, November/December 1995, pp. 1234-1245.
To prevent an open circuit of the load current it or a short circuit of two input phases A, B of a matrix converter from occurring at any time, a defined switching sequence has to be observed. The publication xe2x80x9cA Matrix Converter without Diode Clamped Over-Voltage Protectionxe2x80x9d, J. Mahlein and M. Braun, Conf. Proceed. xe2x80x9cIPEMCxe2x80x9d, 2000, Beijing, China, in particular Chapter 3, describes possible commutation sequences for an output phase of a matrix converter. The commutation from the state semiconductor switches S1 and S2 conducting and semiconductor switches S3 and S4 blocking into the state semiconductor switches S1 and S2 blocking and semiconductor switches S3 and S4 conducting will, now be described based on the figures in the reference, which are reproduced herein as FIGS. 1 and 2.
As seen in FIG. 1, an output phase of a matrix converter 2 has three bidirectional power switches 4 which are arranged in a 3xc3x973 switch matrix. As also seen in FIG. 1, each bidirectional power switch 4 includes two antiserially connected power switches S1, S2 and/or S3, S4 and/or S5, S6, which each include an antiparallel connected diode. The illustrated semiconductor switches S1, S2, S3, S4, S5, and S6 are implemented as Insulated-Gate-Bipolar-Transistors (IGBT). Each of the antiparallel connected diodes forms a component of the associated IGBT module. Each semiconductor switch S1, S2, S3, S4, S5, S6 of the bidirectional power switches 4 of this phase of the matrix converter 2 can be controlled separately and independently. A switch is regarded as being switched on bidirectionally, if both semiconductor switches S1, S2 and/or S3, S4 and/or S5, S6 of a bidirectional power switch 4 are driven. If only one of the semiconductor switches S1, S2 and/or S3, S4 and/or S5, S6 of a bidirectional power switch 4 are driven, then the switch is called a unidirectionally switched-on switch.
FIG. 2 shows all possible commutation sequences for commutating from the state semiconductor switches S1 and S2 conducting and semiconductor switches S3 and S4 blocking into the state semiconductor switches S1 and S2 blocking and semiconductor switches S3 and S4. These possible commutation sequences depend on information about the polarity of the voltage and/or current and can be divided into three groups. The switching sequences that are not marked can be performed only when the polarity of both the voltage and the current are known and are not of technical interest because two pieces of information are required. A second group surrounded by a dashed line is independent of the voltage polarity and only requires information about the polarity of the current. The third group surrounded by a dash-dotted line is independent of the current polarity and only requires information about the polarity of the voltage. These switching sequences are also referred to as voltage-controlled commutation.
The following discussion is limited to voltage-controlled commutation.
If an erroneous measurement of the voltage polarity with voltage-controlled commutation results in a selection of the wrong switching sequence, then a short circuit occurs in the linked input voltage. This does not cause any technical problems as long as the amplitude of the input voltage is smaller than the turn-on voltage of the semiconductor valves in the short current path, which is approximately 10 V when implemented using IGBT""s. The voltage-controlled commutation hence requires a precise measurement technique for measuring the voltage polarity. Typically employed analog measurements of the input voltage which are required for controlling the matrix converter are not adequate, so that additional electronic components are required. The required high precision is also vulnerable and does not lend itself to a desired robust solution of the commutation problem.
The publication xe2x80x9cA New Two Steps Commutation Policy For Low Cost Matrix Convertersxe2x80x9d, M. Ziegler and W. Hofmann, Conf. Proc. xe2x80x9cPCIM 200 Europexe2x80x9d, Nxc3xcrnberg, September 2000, proposes a control method with relaxed requirements for determining the voltage polarity. The control method described in this reference will now be described based on a time-dependent diagram of an output phase with pulse-width modulation as depicted in FIG. 3.
The three-phase matrix converter 2 has nine bidirectional power switches 4, which are arranged in a 3xc3x973 switch matrix 6. The arrangement of the nine bidirectional power switches 4 in a 3xc3x973 switch matrix 6 allows each output phase X, Y, Z to be switched to any desired input phase U, V, W. An inductive load 8 is connected to the output phases X, Y, Z of the matrix switch 2. The input phases U, V and W are connected with an LC-filter 10, which is connected on the input side to a power mains system 12. The LC filter 10 includes inductors 14 and capacitors 16. The capacitors 16 are shown here in a star configuration, although a delta configuration is also possible. The inductors 14 are arranged in the supply lines to the capacitors 16, thereby smoothing the charge currents. One phase of the matrix converter 2 has three bidirectional power switches 4 adapted to connect an output phase X or Y or Z, respectively, with the input phases U, V, W. This matrix converter phase has a 3xc3x971 switch matrix.
The equivalent circuit diagram of the exemplary three-phase matrix converter 2 also shows a control device 18, a modulation device 20 and nine driver devices 22. The control device 18 includes a load regulator 24 and a control set 26. The load regulator 24 receives at its input a measured load current vector io and produces an output voltage vector uo. The load regulator 24 can be, for example, a field-oriented regulator. By applying the space vector modulation method, the control set 26 connected after the load regulator 24 calculates modulation levels m as a function of the generated output voltage vector uo and of a measured input voltage vector ui. The control device 18 is preferably implemented as a digital signal processor.
The modulation device 20 connected after the control device 18 has on the input side a modulator 28 and on the output side for each matrix converter phase a commutation controller 30, 32, and 34. Depending on the applied drive level m, the modulator 28 generates pulse-width-modulated signals which are each checked and processed in the commutation controllers 30, 32, and 34 with respect to blocking time, minimum on period and open time. The commutation controllers 30, 32, and 34 need to know the polarity of the linked input voltages of the matrix converter 2 which are described by an input voltage space vector ui. Control signals (On/Off signals) are provided at the outputs of the commutation controllers 30, 32, and 34, which are converted by a driver device 22 into a gate signal which is independent of the specific embodiment of the bidirectional power switch 4. The modulation device 20 is preferably implemented by a Programmable Logic Device, in particular a field programmable gate array.
According to the diagram of FIG. 3, the control set 26 of the matrix converter 2 supplies a commutation sequence A, B, C for a half modulation period T, with a mirror-symmetric modulation (triangular baseline overshoot method) depicted in FIG. 3. As also seen in the diagram, the potential levels A and B of two input voltages of the matrix converter 2 are very close to each other. I.e., the polarity of the linked voltage between the two input voltages with the potential levels A and B is uncertain, which is detected by the modulator 28. As described above, since an erroneous measurement of the voltage polarity causes a short-circuit of the linked voltage, the method disclosed at the PCIM Conference not commutate directly between the potential levels A and B, and a bypass commutation is performed instead. In other words, the commutation proceeds from the potential level A to the potential level C and immediately thereafter from the potential level B to the potential level C. This is shown in FIG. 3, wherein the predetermined commutation sequence A, B, C for half a modulation period T/2 is augmented by an additional potential level C. This additional potential level C is arranged in a second row, to indicate that this is a bypass commutation which is added only when the polarity of a linked input voltage is uncertain. By using a bypass commutation, the polarity between the potential levels A and C, as well as a between C and B, can be definitely determined without requiring a highly accurate measurement technique.
It should be mentioned that, depending on the switch settings before commutation, the potentials referred to by A, B and C will have to be associated with the actual input potentials UX, UV and UW, so that the description applies to more general situations.
This control method has the disadvantage that two additional commutations are performed during each modulation period T, whereby the additional commutations are performed in the presence of a large difference voltage. Since the magnitude of the commutation voltage reflects the switching losses, the switching losses are significantly increased with this control method, which is disadvantageous for the chip layout and heat sinking. The switching losses also increase due to the greater number of commutations within a modulation period T. As a further disadvantage, this problem, namely the commutation between two close potential levels, is detected only in the modulator 28 or in the commutation control 30 to 34 of the matrix converter 2.
It would therefore be desirable and advantageous to provide an improved method for controlling a matrix converter, which obviates prior art shortcomings and eliminates commutation between two close input voltages.
According to one aspect of the present invention, a method for controlling a matrix converter, with nine bidirectional power switches arranged in a 3xc3x973 switch matrix, includes the steps of determining switching states with associated time periods for each half modulation interval in a calculated commutation sequence depending on a calculated output voltage space vector of the matrix converter; selecting a commutation sequence for a half modulation interval depending on measured input voltages and a predetermined limit value, with the commutation sequence selected so as to prevent commutation between input voltages having close potentials; comparing the selected commutation sequence with a calculated commutation sequence; and if the selected commutation sequence is different from the calculated commutation sequence, rearranging the switching states with the associated time periods so as to make the selected commutation sequence identical to the calculated commutation sequence.
The control method according to the invention for a matrix converter provides a robust voltage-controlled commutation, for which an analog measurement of the input voltages, which is required anyway for controlling the matrix converter, is sufficient. In addition, the control device connected before the modulation device takes care of the commutation problem. As a result, a matrix converter can be operated with the control method of the invention solely based on analog input voltage information and without specifically measuring the input voltages or output currents.
In addition, with the control method of the invention, only four commutations are performed during each modulation period, which significantly reduces switching losses as well as the chip area and facilitates heat sinking.
In addition, the control method of the invention is not limited to a two-stage commutation, and can be universally extended to other commutation methods, in particular to a four-stage commutation.
According to an advantageous embodiment of the invention, the predetermined limit value for recognizing close voltages can be selected so that the selected commutation sequence is valid for an entire input voltage sector. This has the advantage that the regions which are critical for the commutation can be more easily determined. In addition, the predetermined limit value can be selected so that the selected commutation sequence is valid only for a first region about a value of Zero of a linked input voltage. In particular, any desirable commutation sequence can be selected for a region of an input voltage sector that is different from the first region.